Many coronary stenoses are compliant and are capable of changes in caliber. Large coronary artery vasomotion caused by physiologic factors or by vasoactive drugs could have important effects on blood flow through compliant coronary stenoses. In this project, metabolic control and pharmacologic responses of large coronary artery caliber will be studied in chronically instrumented awake dogs using sonomicrometry. The effects of exercise on large coronary artery hemodynamics and diameter will also be studied. Studies will be done of passive changes in the resistance of compliant coronary stenoses in response to changes in intraluminal pressure in a dog model in which the diameter of the artery in the area of the stenosis will be measured. Studies will also be done concerning the responses of compliant coronary stenoses to vasoactive drugs and direct correlations will be made of changes in coronary arterial diameter, stenosis pressure gradient and blood flow through the stenotic artery. The proposed research is planned to contribute to our understanding of the mechanisms by which a coronary stenosis may lead to myocardial ischemia or infarction and to investigate how drugs modify the effects of severe coronary stenoses.